Internal combustion engines play a dominant role in our life; we rely on these fuel-burning machines as a way to provide energy for our everyday needs. Whether recognized or not for our dependence upon them, they're literally one of the most relied upon inventions in the world.
The art and theory behind conventional engines is well-known. Because engines are designed to work in a wide variety of conditions their construction, fuel arrangements, means of ignition, etc., vary. Standard mechanical construction of engines tends to utilize a cylinder with a fit piston which is connected to a crankshaft. Unconventional construction of engines varies greatly; anything from opposed piston designs, to free piston designs, etc., to all kinds of rotary designs which usually have some type of rotor or vane, rather than a conventional piston, to displace a volume. Since an engine's cycle is usually independent of its construction, different construction techniques can be applied to work with a variety of engine cycles (such as 2-stroke cycles, 4-stroke cycles, etc.).
The main goal of an internal combustion engine is to produce useable power efficiently; i.e. to provide the most power while using the least amount of fuel. Special cases exist where other factors such as power to weight, power to displacement or size, or even low manufacturing cost are more sought after than fuel economy (thermal efficiency), but the overall objective is to provide power with fuel efficiency, with other factors considered. That said, the useable energy produced from the best engines running at optimal conditions is only around 25% to 50% of the fuel energy put into the engines. Because of this and because of increasingly tighter regulations on engines, there is an ongoing incentive and effort to raise these numbers and for a wider range past optimal conditions.